1. Field of the Invention
The present invention relates to an acoustic lens composition, ultrasonic probe and ultrasonic diagnostic apparatus.
2. Description of the Related Art
Ultrasonic probes are used in, e.g., fish detectors and ultrasonic diagnostic apparatuses for diagnosing living bodies. These ultrasonic probes use an acoustic lens to increase the resolution by focusing an ultrasonic beam.
In particular, an acoustic lens incorporated into an ultrasonic probe of an ultrasonic diagnostic apparatus for diagnosing a living body, i.e., a medical diagnostic apparatus, is desired to have a convex shape in order to improve the adhesion to a living body. In addition, this acoustic lens is desired to mainly satisfy the following six characteristics.
(1) The acoustic lens desirably minimizes the reflection of an ultrasonic wave from a living body. For this purpose, the acoustic lens is preferably made of a material by which the acoustic impedance (AI=sound velocity×density) of the lens is close to 1.53 MRayls which is the acoustic impedance of a living body.
(2) The acoustic lens desirably transmits and receives an ultrasonic wave at high speed. Therefore, the acoustic lens is preferably made of a material which decreases the attenuation ratio at the use frequency.
(3) To obtain a convex shape, the acoustic lens is desirably made of a material by which the sound velocity in the lens is lower than that (about 1,500 m/s) in a living body. In particularly, the radius of curvature of the acoustic lens is calculated from the relationship between the sound velocity in the lens and that in a living body. This radius of curvature of the acoustic lens can be increased by the use of a material which makes the sound velocity of the lens lower than that in a living body. Consequently, the thickness of the acoustic lens can be decreased.
(4) The acoustic lens is desirably made of a material having good molding properties, particularly, high tear strength. That is, an acoustic lens used in an ultrasonic probe having a central frequency of about 2 to 13 MHz has a thickness of about 0.5 to 1.5 mm. To mold this acoustic lens into a highly precise convex shape, a rubber-based material desirably has high flowability. Especially in an ultrasonic probe whose central frequency exceeds 7 MHz, it is desirable to use a rubber-based material having good molding properties in order to decrease the thickness of an acoustic lens to 1.0 mm or less. Furthermore, acoustic lenses having complicated shapes such as a cap shape are recently often used, so a rubber-based material having not only good molding properties but also high tear strength is desired.
(5) The constituent material, containing additives, of the acoustic lens is desirably harmless to a living body. Also, the acoustic lens is desirably made of a material which is chemically and physically stable in an aqueous glycerin solution, ethyl alcohol, olive oil, or castor oil which is generally used as an acoustic coupling gel or disinfectant.
(6) When in use, the acoustic lens is pushed against a living body with a considerable pressure. If the rubber hardness of the acoustic lens is insufficient, the acoustic lens deforms to shift the focal point, and the image quality degrades. Accordingly, the acoustic lens is desirably made of a material having a durometer A hardness of 50° or more.
The attenuation characteristics of the acoustic lens largely depend not only on the attenuation ratio described in (2) but also on the product of the attenuation ratio and sound velocity. Therefore, the acoustic lens is preferably made of a material which is advantageous in FOM (Figure of Merit). The value of FOM is favorably as low as possible for the same acoustic impedance.
The conventional acoustic lens is made of a rubber-based material obtained by mixing a silica powder in silicone rubber.
Also, Jpn. Pat. Appln. KOKOKU Publication No. 1-34396 discloses an acoustic lens obtained by mixing a predetermined amount of a titanium oxide powder having a predetermined particle diameter in natural silicone rubber.
Jpn. Pat. Appln. KOKOKU Publication No. 5-9039 discloses an acoustic lens composition made up of a silicone rubber compound, an aluminum powder and titanium oxide powder having predetermined particle diameters, and a thermoplastic resin such as nylon having a melting point of 80° C. or more.
Jpn. Pat. Appln. KOKAI Publication No. 8-615 discloses an acoustic lens obtained by adding zinc oxide as a vulcanization assistant to a mixture of silicone-based rubber and butadiene rubber.
In an acoustic lens having a composition obtained by adding a silica powder to silicone rubber, however, the addition amount of the silica powder must be increased to make the acoustic impedance approach 1.53 MRayls which is the acoustic impedance of a living body. The density of a silica powder is about 2.2 g/cm3. To set the density of the whole acoustic lens at about 1.4 to 1.6 g/cm3, therefore, a fine silica powder having an average particle diameter of about 15 to 30 nm, i.e., having a large specific area must be so mixed as to have a weight of about 40 to 50 wt % and a volume of about 24 to 32 vol %. This not only makes incorporation of this amount of the silica powder into silicone rubber difficult, but also makes deaeration after the incorporation difficult. In addition, since the flowability of silicone rubber lowers during molding of the acoustic lens, cracks and pores easily form. The tear strength of the acoustic lens also lowers. On the other hand, although the ratio of attenuation to silicone rubber caused by the addition of this silica powder is relatively low, the sound velocity described in (3) is about 1,000 m/s or more. Consequently, even if the attenuation ratio of the acoustic lens is low, the thickness cannot be decreased. This lowers the value of FOM (attenuation ratio×sound velocity) described above, and increases attenuation.
In the acoustic lenses described in Jpn. Pat. Appln. KOKOKU Publication Nos. 1-34396 and 5-9039, the added titanium oxide powder and alumina powder have the effect of decreasing the sound velocity, when compared to the silica powder. However, to make the acoustic impedances of these acoustic lenses approach 1.53 MRayls as the acoustic impedance of a living body, the addition amounts of titanium oxide and alumina with respect to silicone rubber must be increased as in the case of the silica powder. As a consequence, these acoustic lenses not only increase the attenuation ratio described in (2), but also deteriorate the molding properties described in (4).
Furthermore, the acoustic lens described in Jpn. Pat. Appln. KOKAI Publication No. 8-615 contains a mixture of silicone-based rubber and butadiene-based rubber. This causes swell upon use of olive oil or castor oil as an acoustic coupling material, and lowers the long-term reliability.